8/4/2019 Nitrogen Losses From Soil

1/2

Nitrogen losses from soil

Tri-state Fertilizer Recommendations for Corn, Soybeans, Wheat andAlfalfa, Extension Bulletin E-2567 (New), July 1995

M.L. Vitosh, Michigan State University; J.W. Johnson, The Ohio State University; D.B.

Mengel, Purdue University; Co-editors

Nitrogen (N) can be lost from the field through three principal pathways: denitrifica-tion, leaching and surface volatilization.The form of N a farmer chooses should depend onhow serious a problem he has with the above N losses. Cost of N, labor, equipment andpower availability are other considerations when choosing a fertilizer source.

Denitrification occurs when nitrate N (NO3-) is present in a soil and not enough oxygen(O2) is present to supply the needs of the bacteria and microorganisms in the soil.

If O2 levels are low, microorganisms strip the oxygen from the nitrate, producing N gas

(N2) or nitrous oxide (N2O), which volatilizes from the soil.Three conditions that createan environment that promotes denitrification are wet soils, compaction and warm temper-atures.

Leaching losses of N occur when soils have more incoming water (rain or irrigation)than the soil can hold.As water moves through the soil, the nitrate (NO3-) that is in soilsolution moves along with the water.Ammonium (NH4+) forms of N have a positivecharge and are held by the negative sites on the clay in the soil; therefore, NH4+ forms ofN leach very little in clay.Ammonium forms of N can leach in coarse-textured sands andsome muck soils.These are the only soils where ammonium leaching may be significant.

One way to minimize N leaching and denitrification is to minimize the time the N is inthe soil before plant uptake.This cuts down on the time when conditions are favorable forlosses. Most of the N is needed by corn after the plant is 3 to 4 weeks old (June 1).Surfacevolatilization of N occurs when urea forms of N break down and form ammonia gases andwhere there is little soil water to absorb them.This condition occurs when urea forms ofN are placed in the field but not in direct contact with the soil.This situation can occurwhen urea is spread on corn residues or 28 percent is sprayed on heavy residues of corn-stalk or cover crop.The rate of surface volatilization depends on moisture level, tempera-ture and the surface pH of the soil. If the soil surface is moist, the water evaporates intothe air.Ammonia released from the urea is picked up in the water vapor and lost. On drysoil surfaces, less urea N is lost.Temperatures greater than 50 degrees F and a pH greaterthan 6.5 significantly increase the rate of urea conversion to ammonia gases.Applying urea-type fertilizers when weather is cooler slows down N loss. If the surface of the soil hasbeen limed within the past three months with 2 tons or more of limestone per acre, DONOT apply urea-based fertilizers unless they can be incorporated into the soil.To stopammonia volatilization from urea, the urea must be tied up by the soil.To get the urea indirect contact with the soil requires enough rain to wash the urea from the residue orplacement of urea-based fertilizer in direct contact with soil by tillage, banding or drib-bling. If the residue is light (less than 30 percent cover), 0.25 to 0.5 inch of rain is enoughto dissolve the urea and wash it into the soil. If the residue is heavy (greater than 50 per-cent cover), 0.5 inch or greater of rainfall is required.

8/4/2019 Nitrogen Losses From Soil

2/2

Ammonia volatilization of N may also occur when ammonium forms of Nammoniumsulfate (AS), ammonium nitrate (AN), diammonium phosphate (DAP), monoammoniumphosphate (MAP) and ammonium polyphosphate (APP)are surface applied to calcareoussoils (soil pH greater than 7.5).The extent of loss is related to the reaction productsformed when ammonium fertilizers react with calcium carbonate.

Ammonium fertilizers that form insoluble precipitates (AS, DAP, MAP and APP) are sub-

ject to greater ammonia volatilization losses than AN, which forms a soluble reaction prod-uct.To prevent ammonia volatilization, ammonium fertilizers should be knifed in or incor-porated on calcareous soils.

MSU is an affirmative-action, equal-opportunity institution. Michigan State University Extension programs and mate-rials are open to all without regard to race, color, national origin, gender, religion, age, disability, political beliefs,sexual orientation, marital status, or family status. Issued in furtherance of Extension work in agriculture and homeeconomics, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Tom Coon,Extension Director, Michigan State University, E. Lansing, MI 48824. This information is for education purposesonly. References to commercial products or trade names do not imply endorsement by MSU Extension or biasagainst those not mentioned.